"게가 물에서 편하게 잡니다."
Translation:The crab sleeps comfortably in the water.
You have to memorize which preposition(s) (and whether to use an article) are usable for different location nouns.
As a rule, "by" generally indicates nearness to the object/location in question, while "in" generally indicates being on the interior of something. "The crab sleeps by the water" is a valid sentence, but it means that the crab is near the water, but on dry land, and not surrounded by or touching or floating on the water.
Although "by" is probably without exception in indicating vicinity and not being inside the location or object, other prepositions are used differently for different nouns, and these probably just have to be memorized.
- "On the bus" means inside the bus;
- "On the car" means above, in contact with, and physically supported by the car (what we would call "on top of" the car);
- "On the wall" means in contact with or located at the surface of the wall (usually attached or stuck to the wall, but not always--a red dot of light from a laser could be said to be "on the wall" even though it is not stuck there or supported by the wall). Dirt, writings, hanging pictures, and even intangible visible things like the red dot from a laser can all be said to be "on the wall".
- "On the book" (we would more often say "on top of the book") typically means the same thing as "on the car", that is, above, in physical contact with, and physically supported by the book.
- "On the painting" could mean the same "on" as in "on the book" or "on the car", but it could also mean the same kind of "on" as in "on the wall", depending on what's being described as such. "There is sauce on the painting" means that the sauce is located at the surface of the painting and is presumably sticking to it.
- "In the car" means inside the car in the same way that "on the bus" means inside the bus.
- "In the book" means part of the content of the book. "A castle in the book" refers to a castle described by some part of the book, and may be a fictional castle.
- "In the painting" means the painting portrays the thing described, for example, "There is a castle in the painting" means that the painting depicts a castle. Same with "in the picture" or "in the movie".
- But "in the book" can also just mean in between two of the physical pages of the book. You can often tell which meaning is being used by whether the object in question could conceivably fit between two pages of the book, or if you know the context to know whether the object in question is part of real life. So "there is a castle in the book" means that the book is about or at least mentions a castle, while "there is a bookmark in the book" means that a bookmark is physically between two of the pages of the book. If someone says, "I lost my coin!" and you say, "There is a coin in the book that you were reading earlier, could that be it?" then you know the coin is from real life, not something described by the book. But if someone says, "What if I had a coin that I could use to pay for things, but it would always disappear and return to my pocket?" and you say, "There is a coin just like that in the book I was reading!" then that means that the book describes such a coin in part of the story, since you know a coin like that could not be from real life.